The present invention relates to a double-walled, stackable and unstackable paperboard cup comprising an inner sleeve with a cup bottom, also comprising an outer sleeve with a gap between outer and inner sleeve, also comprising a rolled lip applied to the lower end of the outer sleeve and disposed on the inner sleeve, also comprising a stopping face formed on the inner sleeve for the rolled lip of another paperboard cup to be stacked.
A container of this type is prior art in European patent 1 227 042. A heat-insulating cup is described which comprises a conical inner sleeve and a conical outer sleeve, whereby the inner sleeve comprises an inwardly directed groove, which serves to permit the stacking of an identical cup inside said cup to be stacked. The inwardly directed groove, formed by means of rolling, should serve to provide the cup with good stacking and unstacking properties so that a plurality of stacked cups do not get stuck inside one another. Experience has shown that the stacking properties are satisfactory for approximately 20 cups. If more than this number of cups are stacked together, they become stuck. This is caused particularly by axial pressure, directed from the cup opening to the cup bottom, which is generated by the weight of many cups stacked on top of each other. Even the moderate setting down of 50 packed and stacked cups can result in them becoming stuck to one another. The cause of this getting stuck together must be seen in the insufficient stiffness of the groove, which, however, cannot be improved while applying this method of production, as the rolling results in a weakness in the material.
It is an object of the present invention to significantly improve the stacking and unstacking properties of paperboard cups of the above mentioned type. In particular, in contrast to prior art, a significantly greater number of cups should be stackable, which in particular do not become stuck to one another when a large number of stacked cups are set down with a jolt, or when in any other way a high level of axial pressure acts on the stacked cups, for example when a container magazine is filled. In addition thereto, an improved form stability of the inner sleeve is to be achieved by means of a particular position of the support of the lower rolled lip of the outer sleeve, so that when a cup is being removed from a magazine, it does not stick to the cup into which it is stacked.
This object has been achieved in accordance with the present invention in that the stopping face is designed as a shoulder, below which the diameter of the inner sleeve is reduced discontinuously, and in that the support of the lower rolled lip is applied to the outer surface of the inner sleeve at the same level as, or below the level of, the cup bottom.
The stopping face is formed by a discontinuous reduction in the diameter of the inner sleeve, below which the diameter of the cup remains constant at a certain level. The original conus of the inner sleeve continues again below this cylindrical area. The reduction in diameter is achieved by means of a special forming process, which is described below. By means of the forming process of the stopping face a material strengthening and a material thickening is achieved in the cylindrical area directly below the stopping face, which gives this area an increased stability. The stopping face becomes more resistant to deformation, whereby a high resistance to pressure is achieved. In addition, the angle of inclination of the stopping face, denoted by α in FIGS. 3 and 4, and the depth of the indentation p, influence the stability of the inner sleeve and thus the overall stackability of the cup. In practical embodiments, the depth of the indentation p lies in the range between 0.4 and 1 mm and the angle of inclination α of the stopping face in the range between 20° and 50°. Thus very stable inner sleeves are created, which withstand extreme loads acting in the direction of the cup axis and amounting to more than 200 N, thus avoiding sticking together of the cups.
Even angles of inclination α in the range between 0° and 20° are possible even when the design of the cup is more complicated. The most form-stable stopping surfaces are achieved for these angles of inclination. However, particularly in the case of these embodiments, pressing must take place at increased temperatures, as the inner layer of the inner sleeve would otherwise tear. The inner sleeve of cups of this type are usually made of paperboard, whereby the inner side is covered with a thin layer of a synthetic material. Polyethylene is used in most cases.
If the inner layer is torn, this renders the cup unusable, as it would become moist in the area of the tear due to contact with the liquid therein. An increase in temperature at the form station to a temperature somewhat below the so-called glass transition temperature (softening temperature) of the inner layer fulfills the requirements for making the layer so ductile that even angles of inclination α of the stopping face in the range of between 0° and 20° are possibe without the layer tearing.
In addition, the form stability of the inner sleeve is increased in that the support of the lower rolled lip is applied to the outer surface of the inner sleeve at the level of the cup bottom or below the cup bottom.
The cup can be produced with or without a shoulder in the area of its opening. The application of an upper shoulder results in a greater gap between the inner sleeve and the outer sleeve, which creates a higher thermal insulation. The upper shoulder, however, has no influence on the stacking properties of the cup.
A further improvement in the stacking properties is achieved through the positive fitting of the lower rolled lip with the geometrical form of the stopping face. In a specific step in the production of the outer sleeve, the form of the lower rolled lip is adapted to the form of the stopping face by means of a pressing element. Each stacked cup achieves a very exact fit because of this adaptation of the form of the lower rolled lip, so that very high stacks of cups, which do not tip over, are possible, because their centre of gravity does not travel out over the standing surface, as a result of which a temporary setting down, for example in the case of the filling of magazine, can be carried out without any risk.
In particular the temperature of the liquid which fills the cup is the basis for the insulating properties of the cup. The thickness of the material of the inner sleeve, followed by the size of the gap between the inner sleeve and the outer sleeve and the material thickness of the outer sleeve all determine the decrease in temperature between the liquid in the cup and the hand which holds said cup. In the case of the usual mass per unit area of the paperboard of the inner and outer sleeve, the gap between the inner and outer sleeves measures as a rule approximately 1.2 mm. Thus, when a cup is filled with a liquid having a temperature of 80° C., this permits an outer temperature of below 60° C., which means that the cup can be held in the hand for a longer period of time without causing pain.
As a result of the optimized stiffness of the inner sleeve of a cup according to the present invention, a saving in material of approximately 15% is made, without the cup losing noticeably in stiffness. The reduced insulating properties arising from the economization in material can be compensated for by a gap increase of approximately 0.2 mm between the inner and outer sleeve.
The present invention also relates to a process for making the cups. An inner cup is produced in preliminary procedural steps (not described here) to the stage where it is equipped with an upper rolled lip and a cup bottom.
The application of the stopping face takes place in a forming station which is integrated into the process line for manufacturing the inner cup and which consists of the elements of a container take-up, a core mandrel and a pressing ring. The core mandrel determines the shape and the properties of the stopping face by means of its cylindrical part and the size of its discontinuous change in diameter. In order to apply the stopping face, the inner sleeve is moved into the cup take-up when the forming station is open. The core mandrel and the press ring have been moved apart to such an extent that an inner sleeve can be mounted on the cup take-up. The parts of the forming station subsequently move forward again, that is the core mandrel and the press ring move towards one another, which movement is denoted in FIG. 10 with arrows. When the forming station has moved together so far that the press ring has reached the cylindrical part of the core mandrel, then a positive fit of the press ring, the inner sleeve and the core mandrel is achieved. The geometrical features of the press ring and the core mandrel are adapted in such a way that the press ring forms a cylindrical part of the inner sleeve while the forming station continues to move together, thus forcing a small percentage of the cup sleeve material towards the stopping face. Thus in the closed position of the forming station a material thickening in the cylindrical part of the originally conical inner cup, and a material strengthening in the stopping face, is achieved. This is possible because the fibre alignment of the wall of the inner cup is identical to the direction of movement of the press ring and the materials used are compressible and the fibres of the material can be elongated.
For very defined stopping faces, whose angle of inclination measures less than 20°, the forming station can be heated in order to improve the flowability of the synthetic layer. Temperatures of between approximately 70° C. and 90° C., which can be generated by means of a warm airstream or by heating the station electrically, lead to good ductility and flowability of the inner layer.
In the next manufacturing step the forming station is again moved apart and the inner cup is transferred to other stations in which it is fitted with an outer sleeve, joined and finished.